/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/MeshTangents.h
If tangents are available in the overlays, returns them. If only Normal is available, computes orthogonal basis. Falls back to unit axes if no overlays are available.
void GetTangentFrame ( int32 TriangleID, int32 TriVertexIndex, FVector3f & NormalOut, FVector3f & TangentOut, FVector3f & BitangentOut ) const